1. Field of the Invention
This invention relates to a print device using a half-tone image reproduction method in which output dots are suitably arranged in a recording matrix to digitize a half-tone or gray-scale image for reproduction.
2. Description of the Related Art
In a conventional print device which is not capable of representing a half-tone image on a picture-element basis, the half-tone image data is converted to binary data for reproduction with a systematic dither method, and then printed.
The systematic dither method is defined as follows. A recording matrix comprising a plurality of device picture elements (pixels) is assumed. The output level of the half-tone image data for a portion (image pixel) of an original image which is to be reproduced is compared with the varying threshold level set for each device pixel in the corresponding portion of the recording matrix. If the output level exceeds the threshold level of a device pixel of the recording matrix, a black dot (output dot) is displayed at that device pixel. This method provides high-speed conversion of half-tone image data to binary image data in a simple control system, so that it has been widely used for many print devices.
Further, for the systematic dither method, a dot-concentration type has been frequently used in which a point on the recording matrix is set as a center point and threshold values of the recording matrix, which are set for every picture element, are set at smaller values for picture elements which are nearer to the center point of the recording matrix, so that the output dots are concentrically arranged around the center point. However, if the dot concentration points as described above are merely arranged in the vertical or horizontal directions, a reproduced image will have a striped pattern artifact imposed on it, because human eyes are sensitive to longitudinal or lateral arrangements. In order to avoid this problem, the threshold levels of the recording matrix are frequently set so that the dot concentration points are arranged in a slanting direction relative to the axes of the sheet on which the image will be formed, so that the arrangement of the dot concentration points becomes inconspicuous to human eyes, since human eyes are relatively insensitive to arrangements in the slant direction.
Further, in a print device in which the threshold values of the recording matrix are freely set to any value, a tiling processing has been frequently used. In this tiling process, the threshold values are set so that the output dots form a figure pattern. A reproduced image is filled with the figure pattern using a function of the systematic dither method.
However, in the conventional print device as described above, the setting of the threshold values is conducted in a fixed printing direction of the device at all times. Thus, when an image is printed on a sheet of paper which is longitudinally arranged (portrait), as shown in FIGS. 11A and 11B, the image on the longitudinal sheet differs from the same image when it is printed on a sheet of paper which is laterally arranged (landscape), as shown in FIGS. 11C and 11D.
FIGS. 1A-1C show half-tone image data and the corresponding cell patterns. FIG. 1A represents a half-tone image 200 which is input as print data, FIG. 1B represents a brick type cell pattern 201 which is formed on the basis of a recording matrix, and FIG. 1C represents an oblique-line type cell pattern 202.
FIGS. 11A-11D show print results using the cell patterns shown in FIGS. 1B and 1C when the printing direction is varied. FIGS. 11A and 11B represent print results which are obtained by using the cell patterns 201 and 202, respectively, to print a triangular portion of the half-tone image 200 on a sheet which is placed in the longitudinal direction (hereinafter referred to as the "longitudinal arrangement" or the "portrait arrangement"). FIGS. 11C and 11D represent print results, which are obtained by using the cell patterns 201 and 202, respectively, to print the triangular portion on a sheet which is placed in the lateral direction (hereinafter referred to as the "lateral arrangement" or the "landscape arrangement"). As apparent from comparison among FIGS. 11A-11D, the cell pattern differs by 90.degree. with respect to the direction of the figure of the input print data between the longitudinal and lateral arrangements.